Currently, catalytic material of honeycomb structure (honeycomb catalytic structure) is in use in order to purify the exhaust gases emitted from kinds of engines, etc. This honeycomb catalytic structure has a constitution in which a catalyst layer 15 is loaded on the surfaces of the partition walls 4 which form cells 3 as shown in FIG. 6. Further, as shown in FIGS. 4 and 5, the purification of exhaust gas with such a honeycomb catalytic structure 60 (a honeycomb catalytic structure 11) is conducted by introducing an exhaust gas into the cells 3 of the honeycomb catalytic structure 60 from one end face 9a side of the structure to contact the exhaust gas with the catalyst layer (not shown) loaded on the surfaces of the partition walls 4 and then discharging the exhaust gas outside from other end face 2b side (see, for example, Patent Document 1 and Patent Document 2).
Also, as a diesel particulate filter (DPF) for capturing fine particles contained in an exhaust gas of diesel engine, there is in wide use a wall-flow type filter obtained by plugging either one end of each cell of a honeycomb structure such as mentioned above so that the cell ends at each end face of the honeycomb structure are plugged alternately ordinarily, each end face of the honeycomb structure looks checkerwise after plugging) and thereby allowing the exhaust gas introduced from one end face side to pass through the porous partition walls having function as a filtering layer and discharge outside from other end face side (see, for example, Patent Document 3).
Recently, the present Inventors have made a study in order to apply the above-mentioned wall-flow type structure of DPF to the above-mentioned honeycomb catalytic structure. In this study, it was attempted to conduct the above-mentioned cell plugging like DPF to a honeycomb catalytic structure such as mentioned above so that an exhaust gas can pass through the porous partition walls having a large number of pores and contact the exhaust gas with the catalyst layer loaded on the inner surfaces of partition wail pores to purify the exhaust gas during the passing-through.
In applying a wall-flow structure to the honeycomb catalytic structure, however, the amount of catalyst layer loaded on the inner surfaces of partition wall pores has a large influence on the properties of the honeycomb catalytic structure obtained. Therefore, the determination of the amount of catalyst layer to be loaded has been an important task to be solved. That is, formation of catalyst layer on the inner surfaces of pores results in smaller pores, which invites a serious increase in pressure loss when the amount of catalyst layer loaded is too large. Also, with smaller pores, the flow rate of exhaust gas passing through pores is higher. When the flow rate of exhaust gas is too high, the efficiency of contact between catalyst layer and exhaust gas is lower and no sufficient purification ability is obtained. Meanwhile, in a case where the amount of catalyst layer loaded is too small or introduction of catalyst particles which constitute the catalyst layer into pores is insufficient during the formation of catalyst layer and the catalyst layer is loaded only on the very small area of the inner surfaces of partition wall pores, the exhaust gas is unable to contact sufficiently with the catalyst layer during its passing through pores and no sufficient purification ability is obtained.
In such a honeycomb catalytic structure, the particle diameters of the catalyst particles used in formation of catalyst layer are also an important item to be investigated. Ordinarily, the formation of catalyst layer is conducted by suspension-loading a catalyst metal (erg. Pt) on the surfaces of particles of heat-resistant, inorganic oxide of high specific surface area, such as γ Al2O3 or perovskite or on the surfaces of particles of CeO2, ZrO2 or mixtures thereof having oxygen storability, to prepare catalyst particles, and adhering and loading them to or on the inner surfaces of pores, etc. of partition walls of a honeycomb structure which functions as a substrate of honeycomb catalytic structure. However, for example, when the particle diameters of catalyst particles are too large as compared with the diameters of pores, introduction of catalyst particles into pores during the formation of catalyst layer may be difficult, or an increase in pressure loss may be incurred by clogging of pores by catalyst particles.
Further, when a catalyst layer is loaded on a honeycomb catalytic structure, the catalyst layer tends to stagnate in the narrow portions of partition wall pores. In such portions where the catalyst layer is stagnant, the catalyst is adhered in a larger amount than in other portions but the effective utilization of most catalyst is not attained because the flow of exhaust gas is restricted. In the worst case of pore clogging by catalyst layer, not only the catalyst of clogged portions but also all the catalyst loaded on the inner surfaces of pores communicating with the clogged portions are not used for exhaust gas purification. Such a problem of low catalyst utilization occurs also in the catalyst introduced into the discontinuous pores not extending from one surface side of partition wall to other surface side or into very small pores. This problem is not preferred not only from the standpoint of catalytic activity blut also from the standpoint of effective utilization of noble metal used as a catalyst, and further invites a disadvantage of a rise in pressure loss, of whole honeycomb catalytic structure.
Patent Document 1: JP-A-2003-33664
Patent Document 2: JP-A-2006-51475
Patent Document 3: JP-A-2001-269585